Signaling system



March 23, 1943. M. KATZIN SIGNALING SYSTEM Filed March 7. 1941CZzrriwWwe our D B i r' F 7 INVENTOR L m MofiZZa/zh BY ATTORNEY PatentedMar. 23, 1943 SIGNALING \SYSTEM Martin Katzin, Riverhead, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication March 7, 1941, Serial No. 382,181

8 Claims.

This application discloses a new and improved method of and means forproducing wave energy modulated fully in accordance with signalpotentials and also modulated in accordance with the mean amplitude ofthe signal potentials. At the receiver the benefits of high percentagemodulation are obtained at all times. The modulations in accordance withthe mean amplitude of the signal potentials are derived and used forautomatic gain control purposes to thus restore the derived signalpotentials to their original relative amplitudes. In a preferredembodiment the modulation'of the carrier in accordance with the meanamplitude is in accordance with oscillations the frequency of which is afunction of the mean amplitude of the signal potentials.

In describing my invention reference will be made to the attacheddrawing wherein Figures 1 to 4 depict schematically a system ofcommunication arranged in accordance with my invention. Figure 1illustrates by block diagram a transmitter system including means formodulating a carrier substantially 100% by signaling potentials and alsoimpressing on said carrier modulations in accordance with the meanamplitude of the signal potentials; Figure 2 is a curve showing acharacteristic of a portion of the equipment in Figure 1; Figure 3illustrates by block diagram a receiving means arranged in accordancewith my invention; While Figure 4 is a curve showing a characteristic ofa portion of the equipment in Figure 3.

Referring to Figure 1, M is a source of modulating potential of thedesired type, here shown as a microphone, connected to the input of anaudio frequency amplifier A. The amplifier A may include 'a number oftube stages of amplification. The output of amplifier A is connected tohigh-pass (or band-pass) filter H and to rectifier R. Amplifiedmodulating potentials are thus supplied to the filter H and to the inputof the rectifier R. Amplifier A is provided with automatic-gain-controlmeans controlled from the output of rectifier R through a time constantcircuit r, c. This control means may include means for applyingpotentials over the line AGC to a control electrode in one or more ofthe amplifier tubes of A. This same gain-controlling voltage is alsoused to control the frequency of operation of oscillation generatingmeans in O, as will be described more fully below. The oscillations atthe output of O are passed through low-pass filter L. The outputcircuits of filters H and L are connected together and to modulator D.The amplified modulating potentialstomatic gain control purposes.

and oscillating potentials are supplied to the modulator D. Themodulator D is connected to transmitting means T which includes carrierwave producing means which provides a carrier that is modulated by theoutput of modulator D.

Oscillator O is used to provide the auxiliary, or control, frequencyused at the receiver for au- Thus auxiliary frequency may, for example,occupy a portion of the audio-frequency spectrum below the lowestfrequency of intelligence to be transmitted, say below 30 to 50 cycles.Oscillator 0 may, for example, be of the 'reactancetube controlled typesuch as shown in Seeley Patent No. 2,121,103, June 21, 1938, or of thetype disclosed in Crosby Patent No. 2,065,565, December 29, 1936. Theoscillator O and control means is similar to those used in automaticfrequency controlled receivers. The final control frequency may beobtained most conveniently by having the direct current control voltagefrom rectifier'R, supplied by way of line cafe, control a reactance tubeimpedance or equivalent means to thereby vary the frequency of a highfrequency oscillator in O in accordance with the direct currentpotential variations. These controlled high frequency oscillations arethen heterodyned to the desired very low audio frequency ofcorrespondingly varied frequency, by means of a fixed frequencyauxiliary oscillator. These oscillations of fixed frequency may bederived from the carrier wave frequency generated in transmitter T ormay be generated in a separate oscillator. Filters H and L have theircut-off frequencies at the lowest frequency of intelligence to betransmitted, say 30 to 50 cycles.

The operation of the transmitting arrangement of Figure 1 is then asfollows: Sound waves actuating microphone M are converted'intoelectrical vibrations which are then amplified by audio-frequencyamplifier A. Due to the provision of automatic gain control means, theoutput of amplifier A is maintained substantially constant, so thattransmitter T may be operated at substantially per cent modulationcontinuously.

The direct current AGC voltage which operates to maintain the output ofamplifier A substantially constant also operates oscillation generatingmeans in O to vary the frequency of the control frequency oscillationssupplied from O7 to L in accordance with variations in the volume levelof the sound waves impinging on microphone M. The relation between thedirect current AGC voltage and the control frequency produced by themeans in may be as shown in Figure 2. Curve I shows the controlfrequency increasing uniformly with the AGC voltage. Another possiblecharacteristic is shown by the dashed curve 2 which indicates thecontrol frequency decreasing uniformly with the AGC volt age. Forexample, in the method of obtaining the control frequency as describedabove by heterodyning a controlled high frequency oscillator to thefinal low audio frequency, the characteristic of curve I would beobtained by choosing the heterodyning frequency at the lower end of therange of the high frequency oscillations, while the characteristic ofcurve 2 would be obtained by placing the heterodyning frequency at theupper end of this range.

The receiving device shown in Figure 3 consists of an antenna ANTcoupled to a receiver E comprising radio frequency amplifiers or thesame with heterodyning means with an IF amplifier. The receiver Eincludes a final detector or demodulator which reproduces thetransmitted modulation. The output of receiver E is fed to high-pass (orband-pass) filter H and to low pass filter L. The output of filter H isamplified by audio frequency amplifier A, which is provided withautomatic gain control means, and then reproduced byloud speaker LS.From the output of filter L the oscillations of varying frequencydeveloped in O are passed to a frequency modulated wave demodulator D.This demodulator D may include a discriminator circuit and detectorwherein the frequency variations of the oscillations 0 are changed tocorresponding amplitude variations and demodulated. For example, Iprefer to use here a demodulator of the type disclosed in Crosby PatentNo. 2,229,640, dated January 28, 1941, or Seeley Patent No. 2,121,103,dated June 21, 1938, or Conrad Patent No. 2,057,640, dated October 13,1936. Frequency variation detecting means of the type shown in HansellPatent No. 1,813,922, July 14, 1922 may also be used as the detectingmeans in D. The demodulating means in D translates frequency variationsinto amplitude variations. The output of D isrectiiied by rectifier R,and the resulting direct'current potentials are used to control the gainof amplifier A through time constant circuit TC. The direct currentpotentials developed in r are used to control the gain of one or moretube amplifiers included in amplifier A.

The operation of the system shown in Figure 3 may be described asfollows: Antenna ANT intercepts the radiated signals which are treatedin the usual manner in receiver E up to the point where they arenormally subjected to audio frequency amplification. Following the finaldetector in E the received modulation band is separated into the controlfrequency band and the intelligence band by means of filters L and Hrespectively. The control frequency operates on discriminator D, theoutput of which has an amplitude which is dependent on the value of thecontrol frequency. The output is then rectified by R and used to controlthe gain of amplifier A, which receives the intelligence band offrequencies passed by filter H. Thus, although the transmitter Tern-itssubstantially constant percentage modulation the provision of thecontrol frequency enables the gain of the receiving system to be variedin inverse relation to the volume level of the original sound waves. Thefinal sound waves emitted by LS may therefore be made to be afaithful'reproduction of the original. The characteristic ofdiscriminator D is preferably as shown in Figure 4. Curve l of Figure 4shows the AGC voltage decreasing uniformly with control frequency,corresponding to curve I of Figure 2. Another characteristic is shown bythe dashed line curve 2 of Figure 4, which corresponds to curve 2 ofFigure 2. Here as in Figure 1 the characteristic I or 2 can be obtainedas desired by properly choosing the heterodyning frequency. In thetransmitter the characteristic 2 can be obtained by reversing thepolarity of the control potentials supplied to 0. At the receiver thegain control potentials at the rectifier output may be reversed toobtain the characteristic indicated at 2 of Figure 4.

I claim:

1. The method of signaling which includes the steps of generating acarrier wave, modulating said carrier wave in accordance with signalpotentials, controlling the amplitude of said signal potentials toobtain substantially full modulation of said carrier wave by said signalpotentials, generating alternating potentials the frequency of which isproportional to the mean amplitude of the signal potentials, andmodulating said carrier wave by said generated alternating potentials.

2.v The method of signaling which includes the steps of generating acarrier wave, modulating the amplitude of said carrier wave inaccordance with signal potentials, controlling the amplitude of saidsignal potentials to obtain continuous substantially full modulation ofsaid carrier wave by said signal potentials, generating alternatingpotentials the frequency of which is a function of the mean amplitude ofthe signal potentials, and

modulating the amplitude of said carrier wave by said generatedalternating potentials.

3. The method of demodulating carrier wave energy continuously modulatedsubstantially by signal voltages and also modulated in accordance withpotentials characteristic of the mean amplitude of the said signalvoltages which includes the steps of demodulating said carrier waveenergy to derive components characteristic of both of said modulations,separating the components characteristic of said last modulations fromthe components characteristic of said first modulations, amplifying saidcomponents characteristic of said first modulations and controlling theamplification of said first modulations substantially solely inaccordance with variations of said components of characteristic of saidseparated components characteristic of said last modulations.

4. The method of signaling which includes thesteps of generating acarrier wave-modulatingsaid carrier wave in accordance with signalpotentials, controlling the amplitude of said signal potentials toobtain substantially full modulation of said carrier wave by said signalpotentials, generating oscillations the frequency of which isproportional to the mean amplitude of said signal potentials, modulatingsaid carrier wave by said generated oscillations, receiving said carrierwaves somodulated, demodulating said carrier wave to derive-themodulation components, amplifying the modulation componentscorresponding to said first modulations of said carrier, and controllingsaid amplification in accordance with the modulation componentscorresponding to said last modulation of said carrier.

5. The method of signaling which includes the steps of generating acarrier wave, modulating the amplitude of said carrier wave inaccordance;

with signal potentials, controlling the amplification of said signalpotentials to obtain continuous substantially full modulation of saidcarrier Wave by said signal potentials, modulating the amplitude of saidcarrier wave by voltage the frequency of which is a function of the meanamplitude of said signal potentials, transmitting said modulated carrierwave, receiving said carrier wave so modulated, demodulating saidcarrier wave to derive the signal potentials and the voltages of varyingfrequency, subjecting said voltages of varying frequency to a frequencydemodulation process to derive voltage characteristic of the meanamplitude of said signal potentials, amplifying the derived signalpotentials, and controlling said amplification in accordance with thevoltages derived by said frequency demodulation process.

6. Wave modulating means comprising a source of signal potentials, asource of carrier waves to be modulated, an amplifier includingautomatic gain control circuits coupling said source of signals to saidsource of carrier waves to be modulated, a controlled oscillator forgenerating potentials the frequency of which varies in accordance withthe mean amplitude of said signal potentials, and circuits formodulating said carrier oscillations in accordance with said last namedpotentials.

'7. In a signaling system, a source of signal potentials, a source ofcarrier waves to be modulated, an amplifier including automatic gaincontrol circuits coupling said source of signal potentials to saidsource of carrier waves to be modulated, circuits for generatingpotentials the frequency of which varies in accordance with the meanamplitude of said signal potentials, circuits for modulating saidcarrier wave in accordance with said generated potentials, connectionsfor transmitting said modulated carrier wave, circuits for receiving anddemodulating said carrier Wave to derive the signal potentials and saidgenerated potentials, an amplifier of controllable gain for amplifyingthe derived signal potentials, and circuits for controlling the gain ofsaid last named amplifier in accordance with said derived generatedpotentials.

8. In a system for receiving and demodulating carrier wave energymodulated substantially by signal voltages and also modulated inaccordance with potentials the frequency of which is representative ofthe mean amplitude of the signal voltages, wave receiving amplifying anddemodulating circuits, a first filter coupled to said circuits forselecting components characteristic of the signal voltages, an amplifierof controllable gain coupled to said filter, utilization means coupledto said amplifier, a second filter coupled to said first named circuitsfor selecting components characteristic of the potentials representativeof the mean amplitude of the signal voltages, at frequency discriminatorcircuit coupled to said last named filter and a rectifier coupling saiddiscriminator circuit to said amplifier of controllable gain.

MARTIN KATZIN.

